In a well plunger system of a type utilizing the present invention, the primary focus is on the production of natural gas ("gas"), but the invention is also applicable to well plunger systems where the primary focus is on oil production. Accordingly, the invention is described in association with a well plunger system producing natural gas but the scope of the invention is not limited to such a system. To begin gas production, a well is bored into the earth to facilitate the removal of gas. In many gas wells the relatively low rate of gas flowing into the well is insufficient to expel oil and water that leak into the well. This unwanted oil and water must be removed from the well, otherwise gas production will effectively cease. Plunger systems powered by the force of the gas pressure itself have been used in an attempt to address this problem.
In a typical well plunger system, the well is sealed off from the outside world with a valve and a cylindrical casing in the well. A sales line connects the valve to the remainder of the gas distribution system and a sales meter is connected to the sales line for measuring amount of gas that has passed through the sales line. An opening at the bottom of the casing permits gas to enter the interior of the casing. Closing the valve has the effect of allowing pressure inside the casing to increase. A tube extends from the valve to near the bottom of the casing. A plunger is positioned at or near the bottom of the tube. After a fixed amount of time has past, or after the casing pressure has exceeded a particular threshold value, the valve is automatically opened and the plunger is forced upward due to the built up pressure inside the casing. Ideally, opening of the valve in this manner allows the gas, as well as any oil and water, to be forced up the plunger tube inside the casing by the plunger. As long as the valve is open, more gas, and in many instances oil and water, flow into the plunger tubing below the plunger. Once the plunger reaches the top of the plunger tube, gas flows through or past the plunger into a line. After a fixed amount of time has past, or after the casing pressure has fallen below a particular threshold value, the valve is closed and the plunger returns back down the tubing to a rest position at or near the bottom of the tube.
In known well plunger systems a controller senses the gas pressure in the casing and opens the valve when the pressure exceeds a fixed value or a fixed amount of time has past. These known well plunger systems have a number of problems in the production of natural gas. If a controller blindly opens the valve when the casing pressure is deemed sufficient, or after a fixed amount of time has past since the last cycle, the valve may either be opened too early or too late in the cycle to optimize gas production. If the valve is opened too early, the pressure in the casing is insufficient to force the plunger to completely lift the water and oil out of the well. If this continues it can result in the well becoming filled ("logged") with oil and water and shutting down ("logging off"). In this case, gas production continues to decrease until it ceases, causing an interruption in gas production and a corresponding loss of revenues derived from that well. It is desirable to prevent the logging off of wells.
In the situation where the valve is opened too late, excessive pressures can build up behind the plunger, forcefully impacting the plunger against the top of the casing and potentially causing damage. Even if no damage is done, waiting too long between opening the valve after each cycle means less gas is produced from the well, again resulting in a corresponding loss of revenues derived from that well. In addition, when excessive pressure builds in the casing, the corresponding pressure differential between the tubing line pressure and the sales line pressure becomes great. In this situation when the valve is opened and the plunger rises at high speeds, the gas flow in the sales line exceeds the maximum measurable by the sales meter. Of course, there is a corresponding loss of revenues derived from the well when quantities of gas flow from the well into the sales line without being registered on the sales meter. Accordingly, it is desirable to optimize the amount of time that is allowed to pass between intervals of opening the valve.
Another problem with known well plunger systems is their inability to modify operation to compensate for the variability of the well's ability to produce natural gas over long periods of time or changes in the sales line pressure. Thus, the characteristics of even a once perfectly tuned system change over great lengths of time, causing the interval between opening the valve to be less than optimal. Accordingly, it is desirable to control a well plunger system in a manner that compensates for changing characteristics of the well such as variability in the well's ability to produce natural gas and variations in sales line pressure.
Yet another problem with known well plunger systems is their inability to adjust the gas flow rate in view of excessive tubing line pressure. Even after the initial rush of gas past the sales meter without detection due to a high build up of pressure in the tubing line causing an excessive pressure differential between the tubing line pressure and the sales line pressure, if the tubing line pressure is very high the gas flow rate past the sales meter exceeds the maximum measurable by the meter. Again this situation results in a loss of revenue from the well. Accordingly, it is desirable to have a well plunger system that adjusts the gas flow rate to compensate for high tubing line pressure.